Electrographic image forming apparatuses are configured to form a toner image by supplying toner to the circumferential surface of a photosensitive drum (an image bearing member) and to subsequently transfer the toner image onto a transfer target (e.g., transfer paper or a transfer belt). Further, generally speaking, electrographic image forming apparatuses are configured to, after transferring the toner image, remove any of the toner (which hereinafter may be referred to as “residual toner”) remaining on the circumferential surface of the photosensitive drum by using a cleaning blade made of rubber, for example.
However, at the tip end of the cleaning blade (such a part of the cleaning blade that is in contact with the photosensitive drum), the residual toner accumulates as the number of times an image forming process is performed by the image forming apparatus increases. Further, at the tip end of the cleaning blade, paper powder substances (e.g., a lump of cellulose and/or a lump of a filler) occurring from transfer paper may also accumulate. There is a possibility that these accumulating substances may go through the tip end of the cleaning blade after the image forming apparatus is used for a long period of time, if slippery characteristics between the tip end of the cleaning blade and the circumferential surface of the photosensitive drum are unsatisfactory. More specifically, the tip end of the cleaning blade is abraded after the image forming apparatus is used for a long period of time, if the slippery characteristics between the tip end of the cleaning blade and the circumferential surface of the photosensitive drum are unsatisfactory. As a result, it becomes easy for the accumulating substances to go through the tip end of the cleaning blade. Further, the accumulating substances that have gone through the tip end of the cleaning blade may firmly adhere to the circumferential surface of the photosensitive drum. In particular, when an external additive (e.g., resin beads) is added to toner particles (toner base particles), the residual toner (or the external additive) easily adheres firmly to the circumferential surface of the photosensitive drum.
When the accumulating substances firmly adhere to the circumferential surface of the photosensitive drum, dash marks (white dots or black dots) appear in output images because of the firmly-adhering accumulating substances (the residual toner, in particular). More specifically, the dash marks appear in positions corresponding to the locations where the accumulating substances are firmly adhering. Further, the accumulating substances firmly adhering to the circumferential surface of the photosensitive drum tend to chip the tip end of the cleaning blade and to make the cleaning function insufficient. In particular, an external additive used in the toner as a polishing agent has a high possibility of chipping the tip end of the cleaning blade.
Further, a technique has been proposed (see Patent Literature 1, for example) by which the slippery characteristics between the tip end of a cleaning blade and the circumferential surface of a photosensitive drum are improved by roughening the tip end of the cleaning blade. According to this technique, because the slippery characteristics between the tip end of the cleaning blade and the circumferential surface of the photosensitive drum are improved, it is possible to reduce the amount of abrasion of the tip end of the cleaning blade. Accordingly, it becomes more difficult for the accumulating substances to go through the tip end of the cleaning blade.
In addition, another technique is generally known by which the slippery characteristics on the circumferential surface of a photosensitive drum are improved by using a leveling agent. According to this technique, it becomes easier for the tip end of the cleaning blade to slip on the circumferential surface of the photosensitive drum. In other words, the slippery characteristics between the tip end of the cleaning blade and the circumferential surface of the photosensitive drum are improved. Accordingly, it is possible to reduce the amount of abrasion of the tip end of the cleaning blade. It therefore becomes more difficult for the accumulating substances to go through the tip end of the cleaning blade.